1. Field of the Invention
The present invention relates to a white LED lighting device suitable for application to illumination lamps, and specifically to street lamps installed in public places such as streets and parks. The invention also relates to a light source for white LED lighting, for use as the light source in the aforesaid lighting device.
2. Description of Related Art
Illumination in streets and parks has conventionally been provided exclusively by incandescent lamps, mercury lamps and fluorescent lamps. However, because these light sources have a relatively high power consumption, the utilization of light emitting diodes (LEDs), which consume much less electric power than fluorescent lamps, has been studied as a means of saving energy. Nevertheless, due to some of their characteristics, such as strong directivity (being a point light source) and the glaring quality of their light, LEDs are not regarded as suitable for outdoors and other types of lighting and hence have not become widely popular in such applications. There are, however, some examples of prior art relating to LED lighting devices in application to indoor and outdoor use (see, for example, Reference 1 below).
In response to this situation, the present inventors have previously proposed a novel lighting device which uses LEDs as its light source and which will provide an LED lighting device that will save energy and also be more suitable for streetlights etc. (see, for example, Reference 2 below).
Reference 1:                Japanese Unexamined Patent Application Publication No. 11-213730 (p. 2–3, paragraphs 9–14, FIGS. 1, 2 and 3)        
Reference 2:                Japanese Patent Application No. 2002-007762 (p. 5–7, paragraphs 8–12, FIGS. 1 and 2)        
The lighting device of Reference 1 has a plurality of LEDs and is provided with a hollow light-guide plate directly below these LEDs. It is also provided with a support for supporting this light-guide plate along its center axis. A diffusion layer having diffuse reflection characteristics and diffuse transmission characteristics is formed on an inner surface of the light guiding part of the light-guide plate, and a diffuse reflection layer is formed on a surface of the support. The LEDs are arranged along the upper end of the light guiding part.
However, numerous problems are encountered in connection with illuminance and product cost in the case of this Reference 1 lighting device. Namely, in order to obtain a planar light source from point light source LEDs, such a lighting device requires a special structure, namely: “provision of (i) a light-guide plate for a plurality of LEDs, this light-guide plate having a light guiding portion obtained by forming, on its inner surface, a diffusion layer having diffuse reflection characteristics and diffuse transmission characteristics, and (ii) a diffuse reflection layer parallel to this light-guide plate”. This requirement not only leads to complexity of structure but also results in optical attenuation in the transparent light-guide plate, leading to poor economy since a large number of LEDs is needed to obtain sufficient illuminance. Moreover, a globe to cover these parts is essential for outdoor applications.
On the other hand, in the Reference 2 lighting device (hereinafter termed the “previously proposed lighting device”), a plurality of LED elements are fitted in a distributed arrangement by being removably inserted in a bullet-shaped hollow container-like LED holder unit having a 5-layer structure, said LED holder unit being mounted, integrally and coaxially with a removable lamp base, on a general-purpose socket; wherein electrical connection and disconnection from the lamp base can be achieved by insertion and removal of these LED elements; and a transparent, bullet-shaped cover is removably mounted on the outside of this so as to form an illumination light with an overall bullet-shaped lamp.
Although the previously proposed lighting device having this structure amply achieves its intended object of providing an energy-saving light, a number of problems still remain to be solved. One of these is the risk of excessive stimulation to pedestrians' eyes due to the rather high intensity of individual lights, resulting from a structure in which point light source LED elements are distributed around the periphery of the LED holder unit. Another problem is that because a conventional LED element is highly directive (i.e., light is output over a narrow angle), it is difficult to obtain a wide overall spread of light. In other words, the irradiation range (i.e., the illuminated area) per lighting device is small, with the result that more lighting units have to be used to illuminate a unit area.
The present invention has been made in the light of the above-mentioned facts. It is accordingly an object of the present invention to provide a white LED lighting device such that (i) the lighting unit configuration and design promotes the dispersion of emitted light, (ii) ample illuminance can be guaranteed without increasing the number of LED elements, and (iii) an energy-saving and maintenance-free lighting device can be obtained. It is a further object of the invention to provide a light source for white LED lighting which is optimal for use in said white LED lighting device.